This invention relates to apparatus for forming a relief printing plate from a liquid photo-sensitive composition. More particularly the invention relates to apparatus having a reversible exposure frame which permits exposure of alternate sides of a layer of photo-sensitive composition to actinic rays whereby both the backing and the image in relief can be produced from a single source of actinic rays.
According to conventional practice a printing plate in which the printing image is raised or in relief is produced by placing an image bearing negative upon a plate of glass. Over the negative is placed a transparent protective film and over the film is spread a layer of liquid photo-sensitive composition. Finally a sheet of mylar or other backing material is placed over the liquid composition. Actinic rays are directed on the side of the layer of photo-sensitive composition adjacent to the negative so that those areas of the composition adjacent to clear areas of the negative are exposed to the rays while areas adjacent to opaque areas of the negative are not.
Following exposure of the actinic rays, the resulting product is immersed in a so-called "wash-out" solution. Areas of the photo-sensitive layer which are exposed to the rays are soluble in the solution while areas not exposed to the rays are insoluble. The product is agitated or brushed in the solution in order to cause the soluble component of the photo-sensitive layer to separate leaving a surface in which the printing image is raised or in relief. The thus obtained product is then dried and is subjected to a post-exposure treatment in order to make it suitable for use as a printing plate.
Known apparatuses suitable for carrying out the various steps in the production of a relief printing plate, particularly apparatuses suitable for production on a small scale, have a number of shortcomings. Some require frequent handling during the the time that the image in relief and the base for the plate are formed and in the interval of time between the various steps in the process. Other machines require at least two sources of actinic rays, one for formation of the image in relief and another for formation of the base for the plate. Still others are unwieldy and complicated of operation and can generally only be operated efficiently by skilled operators.